When pumping a gas-containing medium, such as a pulp suspension containing air or another gas, by means of centrifugal pumps, the pumping process is rendered more difficult by the gas being collected centrally in the pump in front of the impeller. A gas bubble formed in that manner thus increases in size until the pump ceases to operate.
In order to eliminate this problem, centrifugal pumps have been designed with impellers, which are provided with openings for evacuating the gas flow from the central portion of the pump and further through evacuation channels out of the pump. According to known techniques, vacuum pumps are generally used for evacuating the gas. The gas to be evacuated is therefore required to be practically free of impurities so as to limit the wear on the vacuum pumps.
Particularly during the pumping of pulp suspensions problems arise by the pulp following along with the gas. This applies particularly to the pumping of pulp at low concentrations. The pulp concentration is normally in the range of from 1 to 20%. A further problem is that the pulp suspension flowing through in this manner cannot be pumped back to the container, to which the pulp pump is connected. Furthermore, normal vacuum pumps cannot manage high temperatures.
One object of the present invention is therefore to solve the aforementioned problems by using an ejector for evacuating the gas from the medium in the pump.